Since the mid-1980s metallocene catalysts have been used in high-pressure reactors-mainly for producing ethylene-backbone polymers including ethylene copolymers with monomers of one or more of propylene, butene, and hexene, along with other specialty monomers such as 4-methyl-1,5-hexadiene. For example U.S. Pat. No. 5,756,608, granted to Langhausen et al., reports a process for polymerizing C2 to C10 1-alkenes using bridged metallocene catalysts. Polypropylene production in high pressure conditions has, however, been seen as impractical and unworkable at temperatures much above the propylene critical point. A process to produce commercially useful polypropylene in a high pressure system would provide advantages, such as increased reactivity, or increased catalyst productivity, or higher throughput, or shorter residence times, etc. Likewise new polypropylene polymers are also in constant need for the preparation of new and improved products. Thus there is a need in the art to develop new processes capable of greater efficiency and manufacture of new polypropylene polymers.
Supercritical propylene polymerization under relatively mild conditions using supported Ziegler-Natta and metallocene catalysts has been described. Likewise, processes for preparing ethylene copolymers with α-olefins in which polymerization is carried out at a pressure between 100-350 MPa and at a temperature from 200-280° C. using a catalyst based on a tetramethylcyclopentadienyl titanium complex is also known.
Olefin polymerization catalysts for use at polymerization temperatures exceeding the melting point temperature and approaching the polymer decomposition temperature are said to yield high productivity have also been described.
Continuous polyolefin production processes using a metallocene catalyst system and maintain at a pressure below the system's cloud-point pressure creating a polymer-rich and a monomer-rich phase and maintain the mixture's temperature above the polymer's melting point have also been described.
Numerous other publications address various aspects of polyolefin polymerization in an attempt to produce either new polyolefin products or to more efficiently produce products that compete with conventional polyolefin products. Nevertheless there is a need to provide processes that mitigate the costs by for example providing polymers that reduce the need for off-line compounding and rheology treatment to adjust the melt characteristics. At the same time, such a process should not compromise product and/or blend performance by providing propylene polymers that have a desirably narrow difference between the melting and crystallization peak temperatures. Process conditions and catalysts that are capable of providing such relatively high molecular weight polypropylene at more cost-effective lower monomer concentrations and milder temperatures and pressures are still needed in the art.